Towards an effective theory of structure formation with new dark matter physics Jesús Zavala Franco (Marie Curie Fellow) Published results mainly in collaboration with: Mark Vogelsberger (MIT, Cambridge) Abraham Loeb (ITC, Cambridge) Matt Walker (Carnegie Mellon University, Pittsburgh) Kris Sigurdson (UBC, Vancouver) Francis-Yan Cyr-Racine (Caltech, Pasadena) Matthew Buckley (Rutgers, Piscataway) Work in progress also with: Torsten Bringmann (UiO, Oslo) Christoph Pfrommer (HITS, Heidelberg) Department of Physics, University of Oslo, November 2014
OUTLINE ● The “standard” dark matter (DM) model - The cold dark matter (CDM) hypothesis - Structure formation in CDM ● Beyond CDM - What do we know about the DM nature? - C lues from dwarf galaxies? - Proof of concept: structure formation in a self-interacting DM Universe ● Concluding remarks
The dark matter hypothesis DM is made of new particles that do not emit electromagnetic radiation at a significant level. Until now, DM is evident only by its gravitational influence Andromeda Credit: NASA/WMAP-9 Science Team Coma cluster Independent astronomical observations suggest that ~80% of the matter in the Universe is dark Luminous matter + Newtonian gravity ≠ dynamics → missing “dark” matter
The “standard model” of structure formation The particle DM hypothesis is the cornerstone of the current theory of the formation and evolution of galaxies Early Universe (t ~ 0.4 Myrs) Credit: ESA and the Planck Collaboration Universe today (t ~ 13.8 Gyrs) 300 Mpc Huchra +12 Cosmic Microwave Background Radiation Andromeda 2MRS galaxy “map”, large-scale structure galactic scales
The “standard model” of structure formation formation of a DM halo DM gravity only Fig. from Mo, Mao and White, 2010 Aquarius project Springel+08 “Eris” simulation Guedes+11 gas and stellar physics formation of a galaxy
The CDM hypothesis In the Cold Dark Matter (CDM) model, DM is a new cold and collisionless particle In CDM, galaxies form in a purely gravitational DM background, i.e., the nature of DM as a particle is irrelevant for galaxy formation and evolution There is however, no strong evidence to support this strong hypothesis A less stringent hypothesis preserves the success of CDM at large scales and predicts a distinct DM phase-space structure at smaller scales Although there is no indisputable evidence that the CDM model is wrong, there are reasonable physical motivations to consider alternatives
Beyond CDM: motivation for additional (i.e. non-gravitational) DM physics in structure formation
DM nature in the early Universe CMB global DM abundance A guiding fundamental principle? Fig. from Buchmüller 12 e.g. a new symmetry, SUSY DM nature ? Big Bang ?? Early Universe dark particles interacting through the weak force ( WIMPs ) “naturally” WIMPs are excellent CDM candidates! give the right DM abundance
DM nature in the early Universe CMB global DM abundance A guiding fundamental principle? e.g. DM is asymmetric as Fig. from Buchmüller 12 normal matter is DM nature ? Big Bang DM DM Early Universe If DM is asymmetric then much stronger interactions than the weak force are needed to reproduce the observed DM abundance Asymmetric DM is an example of an alternative particle model with non-CDM features!
DM nature and the first cosmic structures Is the minimum scale for DM nature (decoupling) galaxy formation set by the Early Universe DM nature or by gas physics halo mass seed ? (or by both)?
DM nature and the first cosmic structures Is the minimum scale for DM nature (decoupling) galaxy formation set by the Early Universe DM nature or by gas physics halo mass seed ? (or by both)? Credit: Max Tegmark How cold is DM? Ultimately constrained by observations Dwarf galaxies Schultz+13 WIMPs m c > 1.3 keV (thermal) m c ~ 100 GeV 5x10 9 M Sun 1M Earth
Structure formation and DM interactions Onset of structure formation Are non-gravitational DM DM nature halo mass interactions irrelevant for seed DM interactions ? galaxy formation?
Structure formation and DM interactions Are non-gravitational DM interactions irrelevant for galaxy formation? Searches in laboratories on Earth Snowmass CF1 Summary 2013 DM particle interactions (weak force) hoped by most detection efforts!! Searches in space Fermi-LAT collaboration 14 Milky Way satellites 1 cm 2 /g ~ 2 barns/GeV
Structure formation and DM interactions Onset of structure formation Are non-gravitational DM DM nature halo mass interactions irrelevant for seed DM interactions ? galaxy formation? DM particle (weak) interactions hoped by most detection efforts!! Does it interact with ordinary matter? c -nucleus interactions extremely low to impact structure information Does it interact with itself (annihilation)? c-c self-annihilation extremely low to impact structure information 1 cm 2 /g ~ 2 barns/GeV nucleon-nucleon elastic scattering: ~10 cm 2 /gr
Structure formation and DM interactions Onset of structure formation Are non-gravitational DM DM nature halo mass interactions irrelevant for seed DM interactions ? galaxy formation? Does it interact with itself (collisions)? Credit: John Wise / KIPAC Bullet Cluster (Clowe +06) s /m < 1.25 cm 2 /gr 200 kpc (Randall+08) 200 kpc nucleon-nucleon elastic scattering: ~10 cm 2 /gr
Structure formation and DM interactions Onset of structure formation Are non-gravitational DM DM nature halo mass interactions irrelevant for seed DM interactions ? galaxy formation? Does it interact with itself (collisions)? Dwarf MW Cluster nucleon-nucleon elastic scattering: cross section / mass [cm 2 /gr] ~10 cm 2 /g !! 30 3 astro constraints 1/3 10 3 1 10 100 velocity dispersion [km/s]
Structure formation and DM interactions Onset of structure formation Are non-gravitational DM DM nature halo mass interactions irrelevant for seed DM interactions ? galaxy formation? Does it interact with itself (collisions)? Dwarf MW Cluster constraints allow nucleon-nucleon elastic scattering: cross section / mass [cm 2 /gr] collisional DM that is ~10 cm 2 /g !! 30 astrophysically significant in the center of galaxies: average scattering rate per particle: core creation 3 in dwarf galaxies astro constraints ~ <1 scatter/particle/t H > 1/3 Neither a fluid nor a collisionless system: ~ rarefied gas (Knudsen number = l mean /L >~ 1) 10 3 1 10 100 velocity dispersion [km/s]
Structure formation and DM interactions Onset of structure formation Are non-gravitational DM DM nature halo mass interactions irrelevant for seed DM interactions ? galaxy formation? Does it interact with itself (collisions)? Dwarf MW Cluster cross section / mass [cm 2 /gr] 30 velocity-dependence motivated by a Yukawa-like new force in the “dark sector” (hidden sector DM) (analogous to Rutherford scattering) e.g. Yukawa-like, Feng+09 core creation 3 in dwarf galaxies astro constraints 1/3 hard sphere Spergel & Steinhardt 2000 Asymmetric DM is a case model for such interactions 10 3 1 10 100 velocity dispersion [km/s]
Beyond CDM: from a purely phenomenological perspective, the CDM hypothesis is just a restricted case of allowed DM microphysics
Clues of new DM physics from dwarf galaxies? Fornax dSph DDO 154 MW satellite (gas poor) Isolated dwarf (gas rich) M VIS ~ 10 8 M Sun M VIS ~ 10 7 M Sun Dwarf galaxies: most DM-dominated systems M DM > 10 M VIS (ordinary matter is less dynamically relevant)
Theoretical modelling of the galaxy population DM gravity only Fig. from Mo, Mao and White, 2010 Aquarius project Springel+08 “Eris” simulation Guedes+11 gas and stellar physics
Observed abundance of isolated dwarf galaxies M h ~4x10 10 M Sun (~dwarf scale) CDM + simple gal. form. models clearly overpredict the abundance of field dwarfs This an unsolved problem within CDM!
The “too big to fail” problem MW-size halo DM-only simulation MW satellites 30 kpc The most massive CDM-MW-subhaloes seem to be too centrally dense to host the MW dSphs (problem extends beyond MW: Ferrero+12, Garrison-Kimmel+14, Unsolved problem in CDM!! Papastergis+14)
Clues of new DM physics from dwarf galaxies? DDO 154 Fornax dSph ● The dwarf-scale challenges could be related to: ● Misinterpretation of observational data (e.g. incomplete reconstruction of the phase-space distribution,...) ● Incomplete knowledge of galaxy formation (e.g. Indirect energy injection into the DM halo by supernovae,...) ● New DM physics : DM might be collisional : SIDM (e.g. hidden sector DM)
Towards an effective theory of structure formation
Proof of concept to avoid CDM challenges: (i) abundance of dwarfs in the field (ii) too big to fail problem (iii) core-cusp problem DM interactions with relativistic particles in the early Universe + DM-DM self-scattering in the late Universe
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